This invention relates generally to communications systems, and more particularly to an apparatus and method for isolating transients associated with a digital subscriber line.
Digital subscriber line (DSL) technology is a communications format designed to allow a customer to transmit analog voice and data signals over a twisted-pair telephone line. DSL technology provides a significant advantage over other digital communications technologies because it supports the simultaneous or near simultaneous transmission of both analog and digital signals. Also, by using existing telephone lines, DSL provides noticeable cost savings over other high-speed transmission alternatives, such as cable modems and fiber to the neighborhood, which require the placement of new high-speed and expensive cabling.
Different protocols may be used to transmit information using DSL technology. For example, the Asymmetric Digital Subscriber Line (ADSL) protocol typically provides larger bandwidth in the downstream direction at the expense of bandwidth in the upstream direction. This generally allows a customer to receive information at a faster rate than the customer can transmit information. Full rate ADSL, sometimes referred to as xe2x80x9cADSL heavy,xe2x80x9d utilizes a splitter at the customer""s premises to separate the higher frequency DSL signals from the lower frequency voice signals. The splitter protects the DSL signals from interference associated with lower frequency voice line transients caused, for example, when a telephone rings. A lower-cost, less complex version of full ADSL is known as xe2x80x9cADSL Lite,xe2x80x9d which reduces or eliminates the need for a splitter at the customer""s premises.
In actual implementation, both full ADSL and ADSL Lite suffer from interference between the analog voice signals and the higher frequency DSL signals. One approach to eliminating this interference is to permanently place a low pass filter in series with each telephone at the customer""s premises. These filters are connected externally to the telephones and are designed to provide minimally acceptable performance over a wide range of telephones.
One problem with this approach is that by designing the filters to work with a wide variety of telephones, it is typically difficult to get optimal or near optimal performance with any particular combination of filters and telephones. Telephone equipment manufacturers develop and produce different types of telephones. A telephone produced by one manufacturer often has a different impedance than other telephones developed by other manufacturers. As a result, different DSL customers may use telephones with widely varying impedances. The varying impedances of the telephones affect the design of the low pass filters because the impedance of the telephone affects how well the filters remove transients from a signal. The low pass filters are typically tuned to a wide range of frequencies so that the filters may be used with the different types of telephones. Because the filters are tuned to a wide range of frequencies, the filters may allow some noise to affect the analog voice or DSL signals communicated over the telephone lines.
Another problem with this approach is that the filters may be unsuitable for certain telephones. If a filter is tuned to a frequency range that is too narrow, the filter may adversely affect the analog voice signals transmitted by the telephone. Beyond that, the filter might actually filter out the analog voice signals. As a result, the filter may be unsuitable for use with telephones that have high or low impedances.
A further problem with this approach is that the filter needs to be physically installed at the customer""s premises. This may increase the expense of providing DSL service to a customer because a technician may need to visit the customer""s premises to install the low pass filter.
In addition, when a customer""s premises have more than one telephone, the filters associated with telephones that are not in use may create resonant circuits and cause interference in telephones that are in use. This interference not only hinders the performance of the telephones, but it may also create irregularities and errors in the higher frequency DSL signals communicated to and from the customer""s premises.
The present invention recognizes a need for an improved apparatus and method for isolating transients associated with a digital subscriber line. The present invention substantially reduces or eliminates problems and disadvantages associated with prior systems and methods.
In one embodiment of the invention, a communication apparatus includes circuitry operable to facilitate communication over a digital subscriber line. The apparatus also includes a filter operable to be coupled between the circuitry and the digital subscriber line. The filter has a corner frequency and is operable to attenuate transients associated with the circuitry when the apparatus is in use. The filter is tuned to an impedance of the circuitry.
In a particular embodiment, the communication apparatus also includes a switch coupled to the filter. The switch is operable to effect a change in the corner frequency of the filter depending on whether the apparatus is in use. The change in the corner frequency operates to attenuate parasitic interference associated with a second apparatus coupled to the digital subscriber line when the second apparatus is not in use.
Numerous technical advantages can be gained through various embodiments of the invention. Various embodiments of the invention may exhibit none, some, or all of the following advantages. For example, in one embodiment of the invention, an apparatus is provided that may be used with a digital subscriber line. In a particular embodiment, the apparatus is a telephone. The telephone includes a filter, which may allow the telephone to be used without causing excessive interference with DSL signals communicated over the digital subscriber line. Also, because the telephone includes a filter, this may help reduce the cost of providing DSL service to a customer. A technician need not physically install a filter at the customer""s premises.
Some embodiments of the invention may also provide a filter that works more effectively. The filter in the apparatus is tuned to the impedance of the apparatus. This may narrow the frequency range of the filter and allow the filter to remove more noise from the voice and DSL signals communicated over the digital subscriber line. Further, because the filter is tuned to the impedance of the apparatus, the invention helps to reduce or eliminate the possibility that a filter will be unsuitable for use with telephones having high or low impedances.
In addition, some embodiments of the invention reduce or eliminate the parasitic interference associated with the use of multiple filters coupled to multiple telephones. For example, the invention can be designed so that the inductance of a filter increases when the telephone is not in use. This reduces the corner frequency of the filter to a point where parasitic interference associated with that filter does not affect other telephones that are in use. Also, the invention can be designed so that the capacitance of the filter increases when the telephone is in use. This also reduces the corner frequency of the filter to a point where the filter operates to attenuate parasitic interference associated with other telephones not in use.
Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.